Silent carrier beta-thalassaemia due to a severe beta-globin mutation interacting with other genetic elements.

Beta-thalassaemia is caused by the presence of two mutated beta-globin genes, one inherited from each parent. We describe two families in which the diagnosis of beta-thalassaemia intermedia was delayed because one of the parents, an obligatory heterozygote, had normal haematological parameters (silent carrier beta-thalassaemia). DNA analysis revealed that these silent carriers were heterozygous for a point mutation in the polyadenylation signal (AATAAA-AATAAG). This defect is known to cause a moderately severe beta-thalassaemia phenotype. In one case, concurrent deletional alpha-thalassaemia was found in the silent carrier, which may have contributed to the mild phenotype. The increasing availability of DNA analysis should allow prompt diagnosis of such cases. Silent carrier beta-thalassaemia presents a diagnostic challenge to the clinician who evaluates children with anaemia.